The pair will appear alongside Suranne Jones and Lesley Sharp in the third series of the ITV crime drama.
Walker will play a woman who is suspected of brutally murdering her estranged mother - the actress is best known for playing Ruth Evershed on spy drama Spooks and has also appeared in Being Human and Luther.
Bennett - best known for her theatre work in Hairspray and End of the Rainbow - will play Sharon, the mother of Rachel Bailey (Jones).
The new character is said to bring "a whirlwind of chaos, upset and embarrassment" into the lives of Rachel and her siblings.
Scott & Bailey was officially picked up for a third series in August, with the new episodes set eight months on from series two's climax.
The second series of Scott & Bailey - written and co-created by Sally Wainwright - concluded in April on ITV1, attracting 6.44 million viewers.
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Watch a trailer for Scott & Bailey below: